A vacuum pump draws air by rotating at a high speed a plurality of rotor blades opposite a plurality of stator blades attached to an inner wall of a housing. To rotate the rotor blades at a high speed, a brushless DC motor having an surface permanent magnet (SPM) design has conventionally been used.
The SPM motor of this type has a rotor with magnets bonded on its surface and therefore requires a bind for preventing the magnets from flying away by the centrifugal force during the high-speed rotation. Moreover, the loss of energy dissipated as heat occurs due to eddy currents in the magnets, because the magnets used for the SPM are large.
Accordingly, introduction of interior permanent magnet (IPM) motors for use in vacuum pumps has been discussed with a view to reducing the amount of magnets, simplifying the shape of the magnets, and reducing the number of components by omitting the bind for preventing the magnets from flying away.
The IPM motor is a synchronous motor with magnets embedded inside the rotor to produce rotating magnetic fields and capable of outputting large power despite the small size thereof, as it can use both reluctance torque generated by rotor magnetization and magnet torque given by the magnets.
To use this IPM motor as the motor for a vacuum pump, a rotor shaft for rotating the rotor blades of the vacuum pump at a high speed needs to be pressed into a rotor part of the IPM motor. The rotor shaft of the vacuum pump needs to have a diameter of a certain value or more so as to reduce natural vibration during high-speed rotation of the vacuum pump, so that, the rotor of the IPM motor in which magnets are embedded has to have a very small yoke width. If the magnet of each pole consists of a single magnet, the rotor yoke width may be too small for the rotor core or magnets to be able to mechanically withstand the pressure during the process of pressing in the shaft of the vacuum pump, and the rotor core or magnets may break.
“Motor rotor” described in Japanese Patent Application Laid-open No. 2002-44887 is known as a rotor for an IPM motor configured with two magnets per one pole.